The demand for communications capacity on fiber optic communications networks has resulted in increased use of wavelength-division multiplex (WDM) systems. WDM systems provide another level of multiplexing to individual or already multiplexed channels. In WDM systems, the individual transmission channels are carried on separate optical wavelengths and are combined in a single fiber at the optical link layer. WDM systems are generally used when the number of fibers in a transmission link is inadequate for the capacity or becomes a critical cost item.
The distances between WDM optical equipment and the signal transmission rates may be attenuation and/or dispersion limited. As a result, amplifiers are employed to boost signal strength to overcome these limitations due to losses from WDM devices, dispersion compensating fibers/gratings, connectors, splices, and fiber. As amplifier designs and WDM systems have evolved, so has the need for increased power grown. Optical power levels generated by fiber optic transmission systems can therefore be high enough to be hazardous to the human eye. If the transmission fiber is disconnected or accidentally cut or broken, the open end of the fiber can be emitting hazardous power, and this emission can possible be pointed into a person's eye. This power is higher when multiple optical channels are transmitted over a single fiber, as in WDM systems. Even higher power levels are generated when optical amplifiers are used.
Fiber optic transmission systems may employ a safety shutdown function which will shut down the optical output when the fiber is disconnected, cut, or broken. Such existing safety shutdown systems are based on the detection of input loss of power, which is then used to shut down the optical output. While the use of input power loss detection for control of safety shutdown is acceptable in certain systems, this method is not sufficiently reliable in WDM systems which are more complex. Added complexities in WDM systems can create, excessive system noise, and/or low levels of signal to noise ratios, which will in turn prevent safety shutdown based on detection of input loss of power. An example of a system with a high level of noise and/or low signal to noise ratio is a fiber optic transmission system which utilizes a Raman amplifier connected to the transmission fiber upstream of the receiver device to provide preamplification. Another example of a system with a high level of noise is one which uses bi-directional fiber operation. This type of system can have significant noise levels at the receiver due to high transmit power and Rayleigh back scatter in the transmission fiber. Even if these conditions do not cause an excessive noise level for preventing detection of a fiber cut or disconnect, when a fiber is broken, there is a significant possibility that the broken end will reflect a significant amount of the transmitted power, such that the receiver will see enough optical power that the receiver will not detect input loss of power, and therefore will not be able to activate the safety shutdown function.
Actuation of safety shut down functions can also fail due to having a fixed level for input loss detection, and received power levels that may be high or low depending upon different installations, transmission cable length, and transmission cable losses. If these variable levels for loss detection and shutdown control are adjustable through software, the failure rates due to software or processor failures maybe too high to meet reliability requirements.
A need has thus arisen for a system and method for reliably actuating a safety shut down function in a fiber optic transmission network where the network may utilize amplifiers such as, for example, Raman amplifiers, erbium-doped fiber amplifiers (EDFAs) and other remote pumped EDFAS for boosting signal strength in WDM systems.